1. Field of the Invention
The present invention relates to a contrast control circuit fit to be incorporated into, for example, a CRT controller.
2. Description of the Related Art
Some CRT monitor which displays pictures represented by signals provided by a computer inserts a reference pulse for contrast control in the back porch of the horizontal blanking interval of a video signal.
Referring to FIG. 4 showing a conventional CRT controller of such a type, an adder 2 inserts a reference pulse of a specified level in the back porch of a video signal in each horizontal scanning cycle, a video frequency amplifier 4 subjects the output of the adder 2 to gain control, a driver amplifier 6 amplifies the video signal provided by the video frequency amplifier 4 and gives its output to a CRT 8. The video signal provided by the video frequency amplifier 4 is applied also to a contrast control circuit 10. The contrast control circuit 10 detects the voltage of the reference pulse inserted in the video signal, compares the detected voltage of the reference pulse with a contrast voltage set by the user, and controls the gain of the video frequency amplifier 4 so that the detected voltage of the reference pulse coincides with the contrast voltage.
FIG. 5 is a block diagram of the contrast control circuit 10 and FIG. 6 is a time chart showing the output signals of the component of the contrast control circuit 10. A synchronous separation circuit 22 extracts the horizontal synchronizing signal from the input video signal and gives the extracted horizontal synchronizing signal to a monostable multivibrator 23. The monostable multivibrator 23 is triggered by the input horizontal synchronizing signal to give a reference pulse to a monostable multivibrator 25. Then, the monostable multivibrator 25 is triggered by the input pulse to give a background pulse as an closing command signal through a buffer 30A to a switch SW1. The reference pulse is given as an closing command signal through a buffer 32A to a switch SW2.
A capacitor C1 is inserted between an input terminal to which the output signal of the video frequency amplifier 4 (FIG. 4) is applied, and the input terminal of the switch SW2. The junction of the capacitor C1 and the switch SW2 is grounded through the switch SW1. A resistor R is inserted between the output terminal of the switch SW2 and one of the input terminals of an operational amplifier OP1. A variable voltage source is inserted between the other input terminal of the operational amplifier OP1 and a ground. The user operates the variable voltage source to set a contrast voltage VR1. A capacitor C2 is inserted between the former input terminal and the output terminal of the operational amplifier OP1.
The output video signal of the capacitor C1 is clamped by the switch SW1 so that the pedestal level is zero while the background pulse is HIGH. The switch SW2 samples the leading edge of the reference pulse inserted in the video signal while the reference pulse is HIGH. Accordingly, the level of the reference pulse is positive with respect to the ground potential for each horizontal scanning cycle.
The operational amplifier OP1 compares the voltage level of the reference pulse sampled by the switch SW2 with the contrast voltage VR1 set by the user and feeds back a voltage to control the gain of the video frequency amplifier 4 so that the voltage level of the reference pulse will coincide with the contrast voltage VR1 to the video frequency amplifier 4.
As shown in FIG. 7, the duration of the back porch must be about 1.6 .mu. sec or above to detect the leading edge of the reference pulse inserted in the video signal while the reference pulse is HIGH by inserting the reference pulse in the back porch and clamping the video signal so that the pedestal level is zero while the background pulse is HIGH. However, the duration of the back porch of some video signal among those used in recent years is less than 1.6 .mu. sec. When such video signals are used, the conventional contrast control circuit shown in FIG. 5 is unable to control the contrast.